A vibration (vibration wave) actuator includes a vibrator that excites drive vibrations to an annular, oblong, or rod-shaped elastic member when an electrical signal, such as an alternating voltage, is applied to an electric-mechanical energy converting element, such as a piezoelectric element. As an example of the vibration actuator, there is proposed a vibration wave motor in which the vibrator is moved relative to an elastic member that is brought into pressure contact with the vibrator.
Now, an annular vibration wave drive device, a stator for a vibration wave motor, a vibration wave motor, and a driving control system are schematically described. The annular vibration wave motor includes an annular piezoelectric element that has an inner diameter and an outer diameter such that the entire circumferential length is equal to an integral multiple of a certain length λ. The piezoelectric element includes two driving regions (drive phases) each having a circumferential length that is equal to an integral multiple of λ/2 along the annular direction. The two drive phases are subjected to polarization processing of reversing the polarity alternately at λ/2 pitches along the annular direction. Therefore, when voltages in the same direction are applied to the drive phases, the piezoelectric element in the drive phases can expand and contract alternately reversely for every λ/2 pitch along the annular direction due to an inverse piezoelectric effect.
The two drive phases are arranged so as to sandwich a non-driving region therebetween, which has a circumferential length that is equal to an odd multiple of λ/4 along the annular direction. In the non-driving region, the piezoelectric element is not subjected to polarization, the piezoelectric element is not applied with a voltage, or the piezoelectric element is subjected to processing that prevents a voltage from being effectively applied thereto. Therefore, the piezoelectric element in the non-driving region cannot actively expand or contract.
The non-driving region may include a detection phase (detection region) for detecting the vibration state of the piezoelectric material. In the detection phase, the piezoelectric element is subjected to polarization processing. Therefore, when a strain is generated in the piezoelectric element in the detection phase by an external force, the piezoelectric element in the detection phase outputs a voltage in accordance with the amount of the strain due to a direct piezoelectric effect.
The vibration wave drive device is obtained by providing electric wire for electric power supply to the drive phase of the piezoelectric element and providing electric wire for voltage detection to the detection phase of the piezoelectric element. A diaphragm formed of an elastic member is bonded to the vibration wave drive device to obtain the stator for a vibration wave motor. As the electric wire, a flexible printed board that is an integrated power feeding member is generally used.
In the stator for a vibration wave motor, when an alternating voltage having a frequency that is the natural frequency of the stator for a vibration wave motor is applied to only one drive phase of the annular piezoelectric element, a standing wave having a wavelength λ is generated in the diaphragm across the entire circumference of the diaphragm along the annular direction. Further, when a similar alternating voltage is applied to only the other drive phase, a standing wave having a wavelength λ is similarly generated in the diaphragm across the entire circumference of the diaphragm along the annular direction. Further, the positions of nodes of the standing waves generated by the two drive phases are shifted from each other by λ/4 along the annular direction of the diaphragm.
Alternating voltages each having a frequency that is the natural frequency of the stator for a vibration wave motor are applied to the two drive phases of the stator for a vibration wave motor in such a manner that the frequencies are the same and the temporal phase difference is π/2. Then, due to synthesis of the standing waves of the two drive phases, a traveling wave having a wavelength λ, which travels in the annular direction, is generated in the diaphragm.
At this time, when focusing on one certain point of the surface of the diaphragm (surface on which the vibration wave drive device is not bonded), a kind of elliptic motion occurs on the surface of the diaphragm. This elliptic motion occurs at all positions of the diaphragm along the annular direction, and hence an object that is held in contact with the diaphragm surface can move along the annular direction of the diaphragm. Further, when the temporal phase difference of the alternating voltages to be applied to the two drive phases is switched to −π/2, the object moving direction is reversed.
By bringing an annular elastic member called a rotor into pressure contact with the surface of the diaphragm of the stator for a vibration wave motor, the vibration wave motor is obtained. The positive or negative sign of the temporal phase difference of the alternating voltages to be applied to the two drive phases of the annular piezoelectric element is switched, and the magnitude of the alternating voltages and the frequency of the alternating voltages are finely adjusted. In this manner, a desired traveling wave can be generated in the stator for a vibration wave motor so as to change the rotational direction, torque, and rotational speed of the rotor.
By connecting a drive circuit to the vibration wave motor, the driving control system is obtained. The drive circuit includes a phase comparator for comparing the phases of the two alternating voltages to output, as a voltage value, phase information based on the result. For example, when the vibration wave motor is driven, the alternating voltage output from the detection phase and the alternating voltage applied to the drive phase are input to the phase comparator. Then, based on the phase difference information output from the phase comparator, a deviation from the resonant state can be known. The electrical signal to be applied to the drive phase is determined based on this information to generate a desired traveling wave. In this manner, the rotational speed of an ultrasonic motor is controlled.
Further, as such an annular vibration actuator, there is known a vibration actuator disclosed in PTL 1.
The ultrasonic motor disclosed in PTL 1 is an annular vibration wave motor including a vibrator that is the stator for a vibration wave motor, which is obtained by bonding an elastic member that is the diaphragm to the vibration wave drive device in which a flexible printed board that is the power feeding member is provided to a piezoelectric body that is the piezoelectric element, and a moving element that is the annular elastic member called the rotor.
The piezoelectric body includes an A-phase electrode that is an A-phase common electrode, a B-phase electrode that is a B-phase common electrode, and an electrode for ground. The flexible printed board includes an A-phase signal line and a B-phase signal line, which are electric wires for supplying electric power to the drive phases, and a ground signal line. Each of the signal lines includes a land portion (exposed portion). The A-phase signal line, the B-phase signal line, and the ground signal line of the flexible printed board are connected to the A-phase electrode, the B-phase electrode, and the electrode for ground of the piezoelectric body, respectively, at the respective land portions.
Further, on a surface of the piezoelectric body on which the A-phase electrode, the B-phase electrode, and the electrode for ground are formed and on a surface thereof on the opposite side across the piezoelectric material, a ground electrode (not shown) is formed. The ground electrode is a common electrode that is electrically connected to the elastic member so as to maintain the surface of the ground electrode of the piezoelectric body to a ground potential through intermediation of the elastic member. The ground electrode is electrically connected to the electrode for ground through intermediation of the elastic member.
The electrode for ground is provided between the A-phase electrode and the B-phase electrode of the piezoelectric body. As disclosed in PTL 1, an interval corresponding to a λ/4 wavelength is provided between an A phase and a B phase that are the drive phases. That is, a region of the piezoelectric body that includes the electrode for ground is the non-driving region that has a λ/4 circumferential length along the annular direction. The electrode for ground and the ground electrode are electrically connected to each other through intermediation of the elastic member, and hence the piezoelectric body between the electrode for ground and the ground electrode is not effectively applied with a voltage.
Further, as such an annular vibration actuator, there is known a vibration actuator disclosed in PTL 2.
The ultrasonic motor disclosed in PTL 2 is an annular vibration wave motor including a rotor and a stator for a vibration wave motor, which is obtained by bonding a metal elastic member 2 that is the diaphragm to the vibration wave drive device in which lead lines and trimming resistors that are the power feeding members are provided to the piezoelectric element having polarized parts (piezoelectric elements 15, 16, and 17 of PTL 2) and an unpolarized part (piezoelectric element 14 of PTL 2).
The drive phase electrodes of the piezoelectric element (piezoelectric elements 15 and 16 of PTL 2) are respectively connected to the trimming resistors that are the power feeding members, and the trimming resistors are respectively connected to the lead lines.
In this case, the trimming resistors are represented by Ra and Rb in the equivalent circuit of FIG. 2 in PTL 2. The trimming resistors are each a thin-film resistor obtained by mixing ceramics with metal, and are each a variable resistor that can be cut by a laser beam. Further, as illustrated in FIG. 3 of PTL 2, the resistance value changes depending on temperature, and is larger than 100Ω.
On a surface of the piezoelectric element (piezoelectric elements 15 and 16 of PTL 2) on which the drive phase electrodes are formed and on a surface thereof on the opposite side across the piezoelectric body, a full-surface electrode (not shown) is formed. The full-surface electrode is a common electrode that is electrically connected to the metal elastic member so as to maintain the metal elastic member to a ground potential through a center portion thereof. Further, the piezoelectric element (piezoelectric element 17 of PTL 2) includes a monitor electrode, and the vibration state of the piezoelectric element (piezoelectric element 17 of PTL 2) can be detected by sandwiching the piezoelectric body with the common electrode.
That is, in the ultrasonic motor disclosed in PTL 2, the piezoelectric elements (piezoelectric elements 15 and 16 of PTL 2) become the drive phases, and another piezoelectric element (piezoelectric element 17 of PTL 2) becomes the detection phase provided in the non-driving region. Further, the unpolarized piezoelectric element (piezoelectric element 14 of PTL 2) also becomes the non-driving region. In view of the drive principle of the annular vibration wave motor and FIG. 1 of PTL 2, the non-driving region of the piezoelectric element (region in which the piezoelectric element 14 is arranged in PTL 2) has a 3λ/4 circumferential length along the annular direction. Further, the non-driving region of the piezoelectric element (region in which the piezoelectric element 17 is arranged in PTL 2) has a λ/4 circumferential length along the annular direction.